The optimum timing of the air intake and exhaust valves of an engine differ according to the running conditions of the engine. At high speed, for example, a large valve lift and a long valve opening period are required in order to obtain large torque, while at low speed, a comparatively small valve lift and short opening period are required. Due to these differences, the running conditions of engines such as car engines vary widely. It was therefore impossible to design a valve drive cam which gives optimum performance for all running conditions.
In Tokkai Sho 63-167016 (Koho) published by the Japanese Patent Office, a variable cam engine is proposed wherein several cams with different shapes are provided, and the optimum valve timing is obtained by selecting these cams depending on the engine running conditions.
In such a variable cam engine, high torque output is obtained from low to high speed by providing a low speed power cam which gives high torque at low speed and a high speed power cam which high torque at high speed, and changing over between the two depending on the engine speed. In addition, an economy cam which has still smaller valve lift and shorter opening period than the low speed power cam has also been proposed to improved fuel cost performance of engine on partial load.
Conventionally, the change-over between these cams was made by simultaneously driving cam selection actuators provided for each cylinder of the engine.
A certain time was however required from the time when a cam change-over command was issued until the time when the change-over was actually completed, and as a change-over could not be performed depending on the range of crankshaft angle, cam change-over was not necessarily completed at the same time for all the cylinders.
It was therefore difficult to specify precisely which cam was being used for each cylinder immediately after issuing a change-over command.
When a cam change-over is made, the air charging efficiency of a cylinder varies according to the cam characteristics even if the throttle opening does not change, so the required fuel quantity and ignition period also vary.
If however it is not possible to specify precisely which cam is being used as described heretofore, it is also impossible to control the fuel supply quantity and ignition period to optimum values. This therefore produced a torque shock after making a change-over and adversely affected the composition of the exhaust gas. This shock had a greater effect for lower engine speeds.
If the change-over were made in synchronization with the engine revolution for each cylinder in turn starting with a specific cylinder, the fuel supply quantity and ignition period could be regulated correctly and the change-over would be smooth. In this case, however, the change-over must be completed within a limited range of crankshaft angles, and especially at high speed, the time period during which the change-over can be made is extremely short. This requires an actuator with a very fast response, high speed input sensing, rapid processing by the CPU and other high performance equipment, therefore even if this could be accomplished it would naturally be very costly.